The Collaborative International Dictionary of English v.0.48:

Culvert \Cul"vert\ (k?l"v?rt), n. [Prob. from OF. coulouere, F.
   couloir, channel, gutter, gallery, fr. couler to flow. See
   Cullis.]
   A transverse drain or waterway of masonry under a road,
   railroad, canal, etc.; a small bridge.
   [1913 Webster]

WordNet (r) 3.0 (2006):

culvert
    n 1: a transverse and totally enclosed drain under a road or
         railway

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "culvert":
   aqueduct, arroyo, bed, canal, creek bed, donga, dry bed, flume,
   gulch, gully, gullyhole, headrace, irrigation ditch, nullah, race,
   river bed, riverway, runnel, sluice, spillbox, spillway,
   stream bed, streamway, swash, swash channel, tailrace, wadi,
   water carrier, water channel, water furrow, water gap, water gate,
   watercourse, waterway, waterworks